There is an age-old question that I have been asked for over twenty years when discussing Wi-Fi, “What is the coverage range of an AP?” The same question is often asked in many other ways, such as “How far will the AP signal travel” or “How far can a client connect to the AP?”
To be honest, this is a question that I have always refused to give an exact answer to because there are too many variables: the attenuation of walls, free space path loss, AP transmit power, and Wi-Fi clients’ receive sensitivity capabilities. In reality, a radio frequency (RF) signal will travel forever in free space, but the signal is constantly losing strength due to the laws of physics.
Every now and then, you may see an AP data sheet with “maximum distance” capabilities and marketing claims of “up to 183 meters (600 feet) in the 2.4 GHz band. These numbers are always misleading because they do not take into consideration the attenuation of walls in an indoor environment. As an RF signal passes through walls, materials will absorb some amount of an RF signal to varying degrees. Brick and concrete walls will absorb a signal significantly, whereas drywall will absorb a signal to a lesser degree. A 2.4 GHz signal will be 1/16 the original power after propagating through a concrete wall. That same signal will only lose one-half the original power after passing through drywall material. The point is that the range of AP depends entirely on the environment in which it is deployed.
The primary coverage goals for any enterprise Wi-Fi network are to provide high data rate connectivity for connected clients and to provide for seamless roaming. A common mistake is to design Wi-Fi coverage based solely on an access point’s capabilities. Truth be told, the effective range of an AP should really be from the perspective of the client devices. In other words, coverage design should be based on the perspective of the Wi-Fi clients. A quality received signal for the client is needed to provide high data rate connectivity and a good user experience. When designing for high data rate connectivity, a –70 dBm or stronger signal is required. Voice-grade Wi-Fi requires a –67 dBm or stronger signal. Also, keep in mind that the effective range for –67 dBm of clients will be less distance than for clients receiving a –70 dBm signal. Remember that for every 3 dB of loss, the received signal is half strength. For example, a –70 dBm signal is half the power of a –67 dBm signal. A client needs to be in closer proximity to the AP for a –67 dBm received signal.
But I thought higher frequency signals attenuate faster?
As shown in Figure 1, the higher the frequency of an RF signal, the smaller the wavelength of that signal. The larger the wavelength of an RF signal, the lower the frequency of that signal.
Figure 1 – Wavelength comparison
Mistakenly, it is often thought that a higher-frequency electromagnetic signal with a smaller wavelength will attenuate faster than a lower-frequency signal with a larger wavelength. However, the frequency and wavelength properties of an RF signal do not cause attenuation. Distance is the main cause of attenuation (and so are thick walls). More importantly, all antennas have an effective area for receiving power, known as the aperture. As previously stated, the perception is that the higher-frequency signal with a smaller wavelength will not travel as far as the lower-frequency signal with a larger wavelength. The reality is that the amount of energy that the aperture of a high-frequency antenna can capture is smaller than the amount of RF energy that a low-frequency antenna can capture. This has an impact on effective range.
A good analogy to a receiving radio would be the human ear. The next time you hear a car coming down the street with loud music, notice that the first thing you hear will be the bass (lower frequencies). This practical example demonstrates that the lower-frequency signal with the larger wavelength is heard from a greater distance than the higher-frequency signal with the smaller wavelength.
Although the RF signals will travel the same distance in the same amount of time, if all other aspects of the wireless link are similar, Wi-Fi equipment using 5 GHz radios will have a shorter effective range and a smaller coverage area than Wi-Fi equipment using 2.4 GHz radios.
But what about 6 GHz?
The introduction of enterprise Wi-Fi 6E APs means the discussion of the effective range of the 6 GHz frequency band is inevitable. The 2.4 GHz band is still be considered a “best-effort” frequency band, and the 5 GHz channels are used for clients that require higher performance metrics. However, the potential of 6 GHz is quite astounding due to all the newly available frequency space (1,200 MHz). As 6 GHz–capable client populations grow, enterprise Wi-Fi networks will also have to be designed for indoor 6 GHz coverage. Vendors are already manufacturing APs with radios for all three frequencies. And therefore, a valid question specific to Wi-Fi 6E is, “Will I have to redesign my network because 6 GHz will not have the same effective coverage range?”
Because of the laws of physics, an electromagnetic signal will attenuate as it travels, despite the lack of attenuation caused by obstructions, absorption, reflection, diffraction, and so on. Free space path loss (FSPL) is the loss of signal strength caused by the natural broadening of the waves, often referred to as beam divergence. RF signal energy spreads over larger areas as the signal travels farther away from an antenna, and as a result, the strength of the signal attenuates. One way to illustrate free space path loss is to use a balloon analogy. Before a balloon is filled with air, it remains small but has a dense rubber thickness. After the balloon is inflated and has grown and spread in size, the rubber becomes very thin. RF signals lose strength in much the same manner. And due to FSPL, an RF signal loses the most power in the first meter it travels.
A decibel (dB) is a logarithmic measure of signal strength gain or loss. Figure 2 shows that a 2.4 GHz signal loses about 40 dB in the first meter. The main reason that the effective range of a 5 GHz AP is much smaller than a 2.4 GHz AP is that the 5 GHz signal attenuates 47 dB in the same first meter. An easier way to explain the difference is that the 5 GHz signal attenuates five times more than a 2.4 GHz signal in the first meter. Luckily, this loss in signal strength is logarithmic and not linear; thus, the amplitude does not decrease as much in the second segment of equal length as it decreases in the first segment. There is a fancy logarithmic equation to calculate free space path loss; however, the 6 dB rule can easily estimate FSPL. The 6 dB rule states that doubling the distance will result in a loss of amplitude of 6 dB, regardless of the frequency. Therefore, at 2 meters, the path loss is 46 dB for 2.4 GHz, 53 dB for 5 GHz, and 55 dB for 6 GHz.
Figure 2 – Free space path loss in the first meter
In the past, coverage planning has been for two bands. For dual-frequency APs, the planning and validation of –65 dBm or –70 dBm coverage was based on the 5 GHz radio. The reason is that the effective range of 5 GHz is much smaller than 2.4 GHz. Therefore, using the lowest common denominator of 5 GHz was preferred when planning for coverage. The good news is that the effective range difference between 6 GHz and 5 GHz is not as significant as the difference between 5 GHz and 2.4 GHz.
On average, a 6 GHz signal attenuates about 2 dB more than a 5 GHz signal in the first meter. Of course, the 6 GHz band is big, so it does depend on what channel is being used. For example, as depicted in Figure 3, in the UNII-5 band of 6 GHz, the path loss in the first meter is about 48 dB, which is only a single dB difference from the average 5 GHz path loss. The first meter path loss in the center of the 6 GHz band is closer to 49 dB, which is 2 dB more loss than a 5 GHz signal.
Figure 3 – Free space path loss in 6 GHz
Whatever the frequency or channel, remember, after the initial first meter, the 6 dB rule states that doubling the distance will result in a loss of amplitude of 6 dB regardless of the frequency.
The bottom line is that the effective range difference between 6 GHz and 5 GHz will not be a serious concern in most indoor Wi-Fi deployments. Current, high-density indoor deployments have already been designed for capacity as opposed to coverage. Of course, the effective range difference between 6 GHz and 5 GHz may have a more significant impact in some verticals, for example, a warehouse environment.
So, will you have to overhaul your Wi-Fi network due to 6 GHz coverage concerns? In most cases, probably not. However, I am a big proponent of proper WLAN planning and design no matter what the frequency. The bulk of troubleshooting calls can be prevented if a WLAN is well planned and designed before deployment. Just as important is a post-deployment validation survey to verify the WLAN design for coverage, capacity and roaming.
Needless to say, it is safe to assume that all the various Wi-Fi predictive modeling solutions such as Ekahau Survey, iBWave Design, TamoGraph Site Survey, and AirMagnet Planner will offer 6 GHz design capabilities in the near future. For Greenfield deployments with tri-band APs that include 6 GHz radios, the lowest common denominator for coverage design will now be 6 GHz.
Portions of this blog have been excerpted from the free eBook: